The present invention relates to a fluid friction clutch of the type having a housing defining a working chamber and a reservoir or storage chamber, primary and secondary parts mounted for relative rotation with respect to each other and exhibiting working surfaces disposed in the working chamber, and a fluid connection for accommodating flow of viscous fluid from the reservoir chamber to the working chamber to control the fluid drive connection between the working surfaces of the primary and secondary parts.
More specifically, preferred embodiments of this invention relate to such fluid friction clutches having a working chamber formed by an output component in wich a driving disk is rotatably arranged, the driving disk being firmly connected with an input component, and wherein said working chamber, for adjustment of the degree of clutching, can selectively be filled with and emptied of a viscous fluid taken from the reservoir chamber by means of a fluid connection existing between the reservoir chamber and the working chamber.
A fluid friction clutch of this type is known, for example, from German Utility Model No. (DE-GM) 81 26 744. For filling the working chamber with fluid, a valve lever is provided that can cover and expose a borehole provided in an intermediate plate separating the reservoir chamber from the working chamber. This valve lever is pivoted in the reservoir chamber and can, at its end that faces away from the borehole, be acted upon by a force via the membrane of a pressure chamber. The introduction of pressure into the pressure chamber therefore causes a force to be exerted on the valve lever, by means of which said valve lever, against the force of a pressure spring, moves away from the borehole between the reservoir chamber and the working chamber and thus clears the path of the fluid from the reservoir chamber into the working chamber. This borehole can therefore be opened or closed through the action of pressure and thus vary the filling of the working chamber with fluid and therefore the degree of clutching. Naturally, fluid return means must also be provided in this case in order to pump the fluid back from the working chamber into the storage chamber. However, the development of these specific fluid return means is not the object of the present invention and are therefore not discussed in detail.
The above-noted type of development of a fluid friction clutch has the disadvantage that for the filling of the working chamber with fluid, movable mechanical parts--especially a valve lever--must be provided. These mechanical parts and in particular, their bearings or support connections, are subject to wear. After a certain time, they therefore have to be exchanged which requires that the whole clutch be removed. The wear limit will be reached especially fast when the filling of the working chamber is controlled through short and frequent pressure surges since the mechanical parts in this case must carry out a large number of movements per time unit.
An object of the invention is to provide a fluid friction clutch of the above-mentioned type that contains few or no moving mechanical parts, in particular, valve parts, and which therefore is not subject to wear. Another objective is to design such a clutch which permits stable intermediate operating speeds.
These objects are achieved according to the invention by developing the reservoir chamber as a pressure chamber, by arranging the opening of the fluid connection on the side of the reservoir chamber at a larger radial distance from the rotational axis of the clutch than the fluid level of the fluid ring formed in the reservoir chamber during the rotation of the output component, and by developing the fluid connection between the reservoir chamber and the working chamber in such a way that an overflowing of fluid from the reservoir chamber into the working chamber cannot take place when the reservoir chamber is without pressure. The reservoir chamber is preferably acted upon by a pneumatic pressure. Via the opening of the fluid connection on the side of the reservoir chamber, fluid will therefore be pumped into the working chamber. The filling of the working chamber is therefore controlled pneumatically, without movable valve parts. The life of this type of clutch is much longer than that of the known embodiments resulting in the saving of maintenance and repair costs. In addition, while the working chamber is being filled with fluid, the pressure existing in the reservoir chamber simultaneously results in a retarding of the return flow from the working chamber into the storage chamber via the return-flow pipe, or other connection so that the filling of the working chamber takes place more rapidly.
For the development of the fluid connection between the reservoir chamber and the working chamber, several advantageous embodiments are contemplated by the present invention. All these embodiments ensure that, when the reservoir chamber is without pressure, no fluid can come into the working chamber from the storage chamber. In one advantageous preferred embodiment, the fluid connection is developed as a pressure control valve. In this case, the radial position of the outlet opening does not affect the functioning.
Another advantageous embodiment provides that the fluid connection is developed as an overflow duct. In order to prevent that fluid flows from the reservoir chamber into the working chamber when the reservoir chamber is without pressure, the outlet opening of this overflow duct on the side of the working chamber is arranged with less radial distance to the rotational axis of the clutch than the fluid ring level formed in the rotating reservoir chamber. This is at least necessary when a fluid with a relatively low viscosity, i.e., good flow characteristics, is used. A different situation arises when highly viscous fluids, such as silicone fluids with viscosity values of over 10,000 cSt are used. Such viscosities are sufficient so that an overflowing of this fluid from the reservoir chamber without pressure into the working chamber will also be prevented when the outflow opening of the overflow duct on the side of the working chamber is arranged at a larger radial distance to the rotational axis of the clutch than the fluid level on the side of the reservoir chamber.
Other advantageous embodiments of the invention provide that the overflow duct connecting the reservoir chamber with the working chamber is developed in such a way that it has increased flow resistance. When the flow resistance is higher than the fluid pressure of the fluid ring forming during the rotation of the output component in the storage chamber, the radial position of the outlet opening on the side of the working chamber is not critical.
In general, the following should be considered concerning the radial position of the outlet opening of the fluid connection on the side of the working chamber when an overflowing of fluid from the reservoir chamber without pressure into the working chamber is to be avoided. The outlet opening of the fluid connection on the side of the working chamber may have a larger radial distance from the rotational axis of the clutch than the fluid level of the fluid ring forming during the rotation of the output component in the storage chamber when:
(a) a pressure control valve is provided;
(b) the fluid has a high viscosity, and/or
(c) the fluid connection has an increased flow resistance.
In other cases, on the other hand, it is required to arrange the outlet opening of the fluid connection on the side of the working chamber in such a way that it has less radial distance to the rotational axis of the clutch than the fluid ring on the side of the reservoir chamber.
An advantageous embodiment of the invention provides that the overflow duct is a pipe which is fastened in a pressure-sealed manner at a wall separating the working chamber from the reservoir chamber, preferably at an intermediate plate. In comparison with the known fluid friction clutches, this embodiment constructively is designed especially simply because in the case of the known clutches, considerable expenditures for components are requred for the movable mechanical valve parts. It is advantageous in this case if the pipe is bent in a U-shaped or V-shaped manner, with both legs pointing away from the rotational axis of the clutch. The fluid located in the storage chamber can therefore only enter into the working chamber through such a pipe when the storage chamber is pressurized. Embodiments are also contemplated where the pipe is shaped differently, where it has to be observed that at least one section of this pipe is at less radial distance from the rotational axis of the clutch than the fluid level in the reservoir chamber so that no fluid can reach the working chamber from the reservoir chamber when the reservoir chamber is not under pressure.
Another advantageous embodiment of the invention provides that the overflow duct consists of a borehole provided in an intermediate plate separating the working chamber from the reservoir chamber. This solution is even more advantageous with respect to manufacturing and expenditure of material. Concerning the development of this borehole, the same considerations apply as in the case of the use of a pipe. It is especially expedient to develop the borehole in a V-shaped manner, the two legs of said V pointing away from the rotational axis of the clutch.
The pressure feed for the reservoir chamber may be developed in a known way. It is especially advantageous to admit compressed air to the reservoir chamber via a pressure feed device arranged on its front side. However, the reservoir chamber may also be supplied with compressed air via a borehole provided in a shaft connected with the input component, according to the contemplated embodiments. The pressure feeding in this case takes place from the direction of the input side and not from the direction of the output side. The shaft connected with the input component may also be led through an intermediate plate separating the storage chamber from the working chamber, by means of a slide ring sealing.
The advantages of a fluid friction clutch constructed according to the invention are especially significant when pressure is admitted to the storage chamber in surges. The reason is that the mechanical valve parts of the known embodiments in this case are subjected to considerable wear. In the case of this method of operation, it is advantageous to control the degree of fluid admission to the clutch working chamber by the variation of the pressure surge frequency and/or the duration of the pressure surges and/or the side of the pressure surges. The cooling water temperature of a motor vehicle may, in this case, for example, be used as a reference control variable.
It is desirable that in the case of an unintentional transfer of pressure from the reservoir chamber, no excess pressure is built up in the working chamber. For this purpose it is advantageous according to certain preferred embodiments to arrange in the working chamber a pressure release borehole at the level of the axis of the clutch on the side of the working chamber that faces away from the reservoir chamber. Since the fluid contained in the working chamber, during rotation of the clutch, forms a ring, it cannot escape through this pressure release borehole. An additional safety means is provided when the pressure release borehole has a relief valve.